1. Field of the Invention
The present invention relates to continuously cast steel slabs for producing hot rolled steel sheets and strips and cold rolled steel sheets and strips including those for plating, which possess excellent workabilities and a production method therefore. More particularly, the object of the present invention is to provide continuously cast steel slabs for producing hot rolled steel sheets and strips and cold rolled steel sheets and strips, including those for plating, having excellent workabilities, such as, deep-drawability, stretchability, bore expansion properties, etc. attained by remarkably lowering the contents of silicon and silica inclusions in the steel.
2. Description of the Prior Art
In the continuous casting of steel, when excessive oxygen is contained in the steel, blow holes are caused which have adverse effects on the continuous casting operation or its products, and thus it is necessary to deoxidize the steel prior to continuous casting.
To remove the oxygen in the molten steel, vacuum degassing and deoxidation by metal deoxidizing agents, such as, Al, Si, etc., are generally used. Also, as for the continuous casting method, various methods have been proposed and disclosed, for example, by U.S. Pat. No. 3,412,781, Japanese Patent Publication Sho 47-47209, Japanese Laid-open Patent specifications Sho 47-31828 and Sho 47-31827. According to the methods disclosed by these prior publications, deoxidation agents are selected and used corresponding to the particular purpose of the steel and complex deoxidation by Al and Si are carried out.
It has been found, however from extensive and various studies conducted by the present inventors, that it is possible to remarkably improve the deep-drawability and stretchability and bore-expansion properties of cold rolled steeel sheets and strips and the bore-expansion properties of hot rolled steel sheets and strips.
Among the various properties and characteristics required for cold rolled steel sheets and strips, excellent deep-drawability and stretchability, in particular, are very important for press-forming, and the bore-expansion property is also an important characteristic for easy bore-expansion working of the opening portions of cans made, for example, from tinned steel sheets. Further, the bore-expansion property is required also for hot rolled steel sheets and strips, and it is very important in the fabricating of motor cases from hot rolled steel sheets, for example, that the joint portions between the cases and other members can be easily bore-expanded.